1. Field of the Invention
The present invention relates to a reflector having a composite reflectivity-enhancing layer as reflecting surface layer on a reflector body where the said composite layer has an outer layer facing the radiation to be reflected, the HI-layer, with a refractive index n.sub.2, and between the reflector body and the outer layer an LI-layer with a refractive index n.sub.1 which is smaller than n.sub.2 and the LI and HI layers are .lambda./4 layers. The invention also relates to the use of such reflectors with reflectivity enhancing composite layer and to a process for its manufacture.
2. Background of the Invention
Reflectors featuring a composite layer system comprising LI/HI-layers deposited on aluminium (LI/HI=Low Refraction Index / High Refraction Index) - i.e. layers exhibiting an inner layer with refractive index n.sub.1 (LI) and an outer layer with a refractive index n.sub.2 which is greater than n.sub.1 - are in general known as reflectors with surface layers that enhance reflectivity.
Such reflectors are normally produced by depositing a very thin layer of high purity aluminium onto the reflector body e.g. of glass or technical grade aluminium (i.e. aluminium of lower purity) e.g. by means of PVD (physical vapor deposition) methods such as sputtering or vaporization. The high purity Al layer is then protected by depositing on it an LI protective layer e.g. made of Al.sub.2 O.sub.3, or SiO.sub.2, usually by PVD or CVD (chemical vapor deposition) methods, and enhanced by a further HI layer to provide a LI/HI reflectivity-enhancing surface on the reflector.
Because of the small thickness of the layer, it is generally not possible to anodise PVD Al layers; consequently, the deposition of the LI and HI layers by PVD or CVD methods is normally carried out under high vacuum. In order to achieve high reflectivity characteristics with composite layers that improve reflectivity, it is necessary to achieve good homogeneity and to keep exactly to narrow, exactly pre-defined tolerances in the thickness of the individual layers. Keeping closely to the exact thickness tolerances of oxide layers deposited in high vacuum using PVD or CVD methods, and checking the thicknesses of these layers is difficult and requires complicated, expensive equipment. The rate of deposition of CVD or PVD layers, especially such dielectric layers, depends on the method used and--compared with chemical methods--is relatively low. In view of the high cost of high-vacuum deposition units, this leads to high manufacturing costs. Furthermore, the low deposition rates and the necessity to use high-vacuum equipment for the PDV or CVD processes makes it difficult or even impossible to produce the layers in a continuous manner.
A further possibility for manufacturing composite layers providing reflectivity-enhancing composite layers is to use chemical or anodic oxidation of aluminium surfaces and subsequently to deposit a dielectric layer with a higher refractive index than aluminium. For that purpose one requires reflectors made of aluminium or reflectors with a layer of aluminium which is thick enough for anodizing. Anodizing is normally performed in a sulphuric acid electrolyte using direct current (dc anodizing). By choosing the appropriate parameters the resultant LI layer can be a homogeneous layer with predefined layer thickness but normally exhibits high porosity which is a result of the process itself. The deposition of the HI layer is normally carried out using PVD or CVD methods. Such reflectivity enhancing composite layers may be produced e.g. in a strip process.